Researchers Find 40 years of Deep Ocean Heating

Greenhouse skeptics often point to the relatively modest atmospheric warming
of the past few decades as evidence of the climatic impotence of greenhouse
gases.

Climate modelers respond that much of the heat trapped by greenhouse gases
should be going into the ocean, delaying but not preventing some of the
atmospheric warming. But oceanographers plumbing the ocean depths have been
unable to say who was right, because records of deep-ocean temperature have been
too spotty to pick out clear trends.

Now, on page

2225 of this issue of
Science, physical oceanographers rummaging through piles of neglected
data report that they have turned up millions of old, deep-ocean temperature
measurements, enough to draw up oceanic fever charts that confirm the climate
models' predicted ocean warming.

"We've shown that a large part of the 'missing warming' has occurred in the
ocean," says oceanographer Sydney Levitus, the lead author of the paper. "The
whole-Earth system has gone into a relatively warm state."

The international data search-and-rescue effort "adds credibility to the
belief that most of the warming in the 20th century is anthropogenic," says
climate modeler Jerry D. Mahlman of the National Oceanic and Atmospheric
Administration's (NOAA's) Geophysical Fluid Dynamics Laboratory in Princeton,
New Jersey. It also suggests that past greenhouse gas emissions guarantee more
global warming ahead and that the climate system may be more sensitive to
greenhouse gases than some had thought.

How could millions of valuable oceanographic measurements go missing for
decades? Oceanographers have never had the orchestrated, worldwide network of
routine observations that meteorologists enjoy. Instead, 40 or 50 years ago,
ocean temperature profiles made by dropping a temperature sensor down through
the sea might end up handwritten on paper, captured in a photograph, or recorded
in analog form on magnetic tape. Everything from mold to mice was devouring the
data. That's why, under the auspices of the United Nations-sponsored Global
Oceanographic Data Archeology and Rescue project, data archaeologists like
Levitus have spent the past 7 years seeking out ocean temperature data around
the world and digitizing them for archiving on modern media.

After adding 2 million profiles of ocean temperature to the previously
archived 3 million profiles, Levitus and his NOAA colleagues in Silver Spring,
Maryland, could see a clear change. Between 1955 and 1995, the world ocean--the
Pacific, Atlantic, and Indian basins combined--warmed an average of 0.06ºC
between the surface and 3000 meters. That's about 20 1022 joules of
heat added in 40 years, roughly the same amount the oceans of the Southern
Hemisphere gain and lose each year with the change of seasons. Half the warming
occurred in the upper 300 meters, half below. The warming wasn't steady, though;
heat content rose from a low point in the 1950s, peaked in the late '70s,
dropped in the '80s, and rose to a higher peak in the '90s. All three ocean
basins followed much the same pattern.

These rescued data have oceanographers excited. "I've never seen anything
like this before," says physical oceanographer Peter Rhines of the University of
Washington, Seattle. "What surprises me is how much [of the warming] is in the
deepwater." The newly retrieved data "show how active the [deep-ocean] system
is," says oceanographer James Carton of the University of Maryland, College
Park, "and how it's a part of the climate system on short time scales."

The friskiness of the whole-ocean system came as a surprise as well. "There's
striking variability from decade to decade," says Rhines. That the heat content
tends to rise and fall in concert across all three ocean basins, in both the
north and the south, is "quite amazing," he adds. Meteorologists and
oceanographers are increasingly recognizing that the atmosphere connects ocean
basins (Science, 10 July 1998, p.

157), but as to what could be
coordinating global swings in heat content, "I really don't know," says Rhines.

The most immediate reward for retrieving so much data from the
oceanographers' attic seems to be more confidence in climate models. The
increased heat content of the world ocean is roughly what climate models have
predicted. "That's another validation of the models," says climatologist Tom
Wigley of the National Center for Atmospheric Research in Boulder, Colorado.

As the models implied, rising ocean temperatures have delayed part of the
surface warming, says climate modeler James Hansen of NASA's Goddard Institute
for Space Studies in New York City, but that can't continue indefinitely. Even
if rising concentrations of greenhouse gases could be stabilized tomorrow,
Hansen says, gases that have already accumulated will push surface temperatures
up another half-degree or so.

The ocean-induced delay in global warming also suggests to some
climatologists that future temperature increases will be toward the top end of
the models' range of predictions. Mainstream climatologists have long estimated
that a doubling of greenhouse gases, expected by the end of the 21st century,
would eventually warm the world between 1.5º and 4.5ºC. Some greenhouse
contrarians have put that number at 1ºC or even less. Now, the ocean-warming
data "imply that climate sensitivity is not at the low end of the spectrum,"
says Hansen. He, Wigley, and some others now lean toward a climate sensitivity
of about 3ºC or a bit higher. But as climatologist Christopher Folland of the
Hadley Center for Climate Prediction in Bracknell, United Kingdom, notes, the
considerable variability in ocean heat content from decade to decade means
scientists will still be hard pressed to find a precise number for climate
sensitivity.

Getting better numbers for ocean heat content remains a top priority for
oceanographers. "There's still a vast amount of data out there that needs
digitizing," says Folland. And for future numbers, an international effort
called Argo, now under way, will create an ocean-spanning network of 3000
free-floating instrument packages. Linked by satellites, the Argo drifters will
create a "weather map" of the ocean down to 1500 meters. At least future
oceanographers won't have to rummage through the data detritus of their
predecessors to see what the ocean is up to.

World's Oceans Warming Up, Could Trigger Large Climate Changes

By Cat Lazaroff

WASHINGTON, DC, March 24, 2000 (ENS) - The oceans of the world have
warmed substantially during the past 40 years, the National Oceanic and
Atmospheric Administration announced Thursday. NOAA researchers suggest that
much of the heat from global warming may have been stored in the oceans,
reducing atmospheric temperature increases but leading to potentially huge
climate changes in the near future.

Researchers from NOAA's Ocean Climate Laboratory in Silver Spring, Maryland
examined three major ocean basins - the Atlantic, Indian and Pacific.

They found the greatest warming has occurred in the upper 300 meters (975
feet) of the ocean waters. This level has warmed an average of 0.56 degrees
Fahrenheit. The water in the upper 3,000 meters (9,750 feet) of the world's
oceans has warmed on average by 0.11 degrees Fahrenheit.

These findings represent the first time scientists have quantified
temperature changes in all of the world's oceans from the surface to a depth of
3,000 meters.

"Since the 1970s, temperatures at the earth's surface have warmed, Arctic sea
ice has decreased in thickness, and now we know that the average temperature of
the world's oceans has increased during this same time period," said NOAA
Administrator D. James Baker.

The ocean and atmosphere interact in complex ways to produce Earth's climate.
Owing to its large mass, the ocean acts as the memory of the earth's climate
system and can store heat for decades or longer.

As a result, it might become possible some day for scientists to use ocean
temperature measurements to forecast the earth's climate decades in advance, the
researchers said.

"It is possible that ocean heat content may be an early indicator of the
warming of surface, air and sea surface temperatures more than a decade in
advance," said Sydney Levitus, who heads NOAA's Ocean Climate Laboratory.

"For example, we found that the increase in subsurface ocean temperatures
preceded the observed warming of surface air and sea surface temperatures, which
began in the 1970s," Levitus said.

"Our results support climate modeling predictions that show increasing
atmospheric greenhouse gases will have a relatively large warming influence on
the earth's atmosphere," Levitus warned.

"One criticism of the models is that they predict more warming of the
atmosphere than has been actually observed. Climate modelers have suggested that
this ‘missing warming' was probably to be found in the world ocean. The results
of our study lend credence to this scenario," he explained.

The scientists determined their findings by using data - 5.1 million
temperature profiles - from sources around the world, to quantify the
variability of the heat content (mean temperature) of the world’s oceans from
the surface through 3000 meter depth for the period 1948 to 1996.

The researchers looked at temperature changes in the Atlantic, Indian and
Pacific oceans.

"In each ocean basin substantial temperature changes are occurring at much
deeper depths than we previously thought. This is just one more piece of the
puzzle to understanding the variability of the earth's climate system," said
Baker.

The Pacific and Atlantic Oceans have been warming since the 1950s, while the
Indian Ocean has warmed since the 1960s. The similar warming patterns of the
Pacific and Indian Oceans suggest that the same phenomena is causing the changes
to occur in both oceans.

The world ocean warming is likely due to a combination of natural
variability, such as the Pacific Decadal Oscillation, and human induced effects,
the researchers say. The scientists, led by Levitus, report their findings in
today’s issue of the journal "Science," in an article titled "Warming of the
World Ocean."

The NOAA report was made possible in part by an international ocean data
management project headed by Levitus that has added more than two million
historical temperature profiles to electronic archives during the past seven
years.

"International cooperation in building the global ocean databases required
for understanding the role of the ocean as part of the earth's climate system
has been excellent," said Levitus.

Contributions of subsurface ocean temperature data have come from all
countries that make oceanographic measurements including the United States,
Russia, the United Kingdom, Germany, France, Canada, Australia, and Japan.

Nearly all of the data were gathered by research ships, naval ships, buoys,
and merchant ships. Some merchant ships deploy instruments that measure the
temperature of the upper ocean as participants in voluntary programs.

Understanding the role of the ocean in climate change and making 10 year
climate forecasts will soon be greatly enhanced by observations planned as part
of an emerging international Global Ocean Observing System.

Meanwhile, a recently completed study of climate over the past 100 years
suggests that interactions between the atmosphere, ocean and sea ice systems may
have played a prominent role in the global warming of the early 20th century,
NOAA scientists say.

Using climate models run on high performance supercomputers, scientists at
NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, conducted
six experiments to explore possible causes for the warming in the first half of
the century. Their findings were also published in today’s issue of "Science."

They linked warming in the early part of the century with a combination of
ingredients, including increasing concentrations of greenhouse gas and sulfate
aerosols.

The supercomputers turned up strong evidence that warming in the latter part
of the 20th century was due in large part to human generated greenhouse gases.

"The fact that all experiments capture the warming from 1970 on is indicative
of a robust response of the climate model to increasing concentrations of
greenhouse gases," said Thomas Knutson, a research meteorologist.

Researchers Find Ocean Temperature Rising, Even in the Depths

By William K. Stevens, The New York Times, March 24, 2000

An important piece of the global-warming picture has come into clearer focus
with a confirmation by scientists that the world's oceans have soaked up much of
the warming of the last four decades, delaying its full effect on the atmosphere
and thus on climate.

The warming of the deep oceans had long been predicted, and the consequent
delaying effect long thought to exist.

But until now the ocean's heat absorption had not been definitively
demonstrated, and its magnitude had not been determined.

The finding, by scientists at the

National
Oceanographic Data Center in Silver Spring,
Md., is based on an analysis of 5.1 million measurements, by instruments around
the world, of the top two miles of ocean waters from the mid-1950's to the
mid-1990's.

The analysis, the first on a global scale, is being published in the March
24, 2000 issue of the journal Science.

As the earth warms, from either natural or human causes, or both, not all the
extra heat goes immediately into the atmosphere, where its effect on climate is
most direct.

Much of it is absorbed by the oceans, which store it for years or decades
before releasing it.

This means that to whatever extent the planet is being warmed by emissions of
greenhouse gases like carbon dioxide, which are produced by the burning of coal,
oil and natural gas, only part of that heating has materialized so far at and
above the earth's surface.

Some experts believe that about half the greenhouse warming is still in the
oceanic pipeline and will inevitably percolate to the air in the decades just
ahead.

The average surface temperature of the globe has risen by about 1 degree
Fahrenheit over the last 100 years. Over the last 25 years, the rate of surface
warming has accelerated, amounting to the equivalent of about 3.5 degrees a
century.

By comparison, the world is 5 to 9 degrees warmer now than in the depths of
the last ice age, 18,000 to 20,000 years ago.

Scientists generally agree that it is unclear how much of the warming is
attributable to greenhouse gases and how much to natural causes; many think both
are involved.

The new study shows that the average warming of the seas over the 40-year
study period amounted to about one-tenth of a degree Fahrenheit for the top 1.9
miles of ocean water as a whole, and more than half a degree in about the top
1,000 feet.

It is possible that the ocean may now be giving up to the atmosphere some of
the heat it stored in the early part of the study period, but this has not been
established, said Sydney Levitus, the chief author of the study. He is the
director of the Ocean Climate Laboratory, part of the data center at Silver
Spring, which in turn is part of the National Oceanic and Atmospheric
Administration.

Likewise, Mr. Levitus said, it is possible but not established that more
frequent appearances of the phenomenon known as El Niño, a semi-periodic warming
of the eastern tropical Pacific that disrupts weather around the world, are
related to the generally warming ocean.

The magnitude of the oceanic warming surprised some experts. One, Dr. Peter
Rhines, an oceanographer and atmospheric scientist at the University of
Washington in Seattle, said it appeared roughly equivalent to the amount of heat
stored by the oceans as a result of seasonal heating in a typical year.

"That makes it a big number," he said.

Dr. James E. Hansen, a climate expert at the NASA Goddard Institute for Space
Studies in New York, said the finding was important because, "in my opinion, the
rate of ocean heat storage is the most fundamental number for our understanding
of long-term climate change."

Three years ago, Dr. Hansen and colleagues used a computer model to calculate
the amount of warming that should have been produced up till then by external
influences on the climate system like greenhouse gases and solar radiation.

They found that because of the storage of heat in the ocean, only about half
the surface warming should have appeared.

Mr. Levitus and his fellow researchers say in their paper that their findings
support the Hansen conclusion.

Still, Mr. Levitus said the cause of the oceanic warming was not clear,
although "I believe personally that some of it is due to greenhouse gases."

Some scientists believe that natural factors like recurring oscillations in
ocean surface temperature in various parts of the world may play a role in the
last century's warming. For example, studies by Dr. Gerard Bond of Columbia
University's Lamont Doherty Earth Observatory found that the climate of the
North Atlantic region, at least, had alternated between cooler and warmer every
1,500 years, more or less.

The world may be entering one of the natural warming cycles now, say Dr. Bond
and Dr. Charles D. Keeling, a climate expert at the Scripps Institution of
Oceanography in San Diego.

In a study published this week in the online edition of Proceedings of the
National Academy of Sciences, Dr. Keeling suggested that a natural fluctuation
in ocean tides over hundreds of years might contribute to these long-term cycles
of warming and cooling.

WASHINGTON - Scientists have discovered a significant, surprising warming of
the world's oceans over the past 40 years, providing new evidence that computer
models might be on target when they predict the Earth's warming.

The broad study of temperature data from the oceans, dating to the 1950s,
shows average temperatures have increased more than expected - about half a
degree Fahrenheit closer to the surface, and one-tenth of a degree even at
depths of up to 10,000 feet.

The findings, reported by scientists at the National Oceanic and Atmospheric
Administration, also might explain a major puzzle in the global warming debate:
why computer models have shown more significant warming than actual temperature
data.

Global warming skeptics contend that if the computer models exaggerate
warming that already has occurred, they should not be trusted to predict future
warming. The models have shown higher temperatures than those found in surface
and atmospheric readings. But now, the ocean data may explain the difference,
scientists said.

In the administration study, scientists for the first time have quantified
temperature changes in the world's three major ocean basins.

''We've known the oceans could absorb heat, transport it to subsurface
depths, and isolate it from the atmosphere. Now we see evidence that this is
happening,'' said Sydney Levitus, chief of the agency's Ocean Climate Laboratory
and principal author of the study.

Levitus and fellow scientists examined temperature data from more than 5
million readings at various depths in the Pacific, Atlantic, and Indian oceans,
from 1948 to 1996.

They found the Pacific and Atlantic oceans have been warming since the
mid-1950s, and the Indian Ocean since the early 1960s, according to the study
published today in the journal Science.

The greatest warming occurred from the surface to a depth of about 900 feet,
where the average heat content increased by 0.56 degrees Fahrenheit. Water as
far down as 10,000 feet was found to have gained on average 0.11 degrees.

''This is one of the surprising things. We've found half of the warming
occurred below 1,000 feet,'' Levitus said. ''It brings the climate debate to a
new level.''

Warming of the World Ocean

Science, March 23, 2000

Sydney Levitus, * John I. Antonov, Timothy P. Boyer, Cathy
Stephens

We quantify the interannual-to-decadal variability of the heat content (mean
temperature) of the world ocean from the surfacethrough 3000-meter
depth for the period 1948 to 1998. The heatcontent of the world
ocean increased by ~2 × 1023 joules between the mid-1950s and
mid-1990s, representing a volumemean warming of 0.06°C. This
corresponds to a warming rate of0.3 watt per meter squared (per unit
area of Earth's surface).Substantial changes in heat content
occurred in the 300- to 1000-meterlayers of each ocean and in depths
greater than 1000 meters ofthe North Atlantic. The global volume
mean temperature increasefor the 0- to 300-meter layer was 0.31°C,
corresponding to anincrease in heat content for this layer of
~1023 joules between the mid-1950s and mid-1990s. The Atlantic
andPacific Oceans have undergone a net warming since the 1950s
andthe Indian Ocean has warmed since the mid-1960s, although
thewarming is not monotonic.

1), the World Climate
Research Program CLIVAR (2), andthe U.S. National
Research Council (3) have identifiedthe
role of the ocean as being critical to understanding the variabilityof Earth's climate system. Physically we expect this to be sobecause of the high density and specific heat of seawater. Watercan store and transport large amounts of heat.

Simpson (

4) conducted the first study of Earth's heat balance which
concluded that the Earth system is not in localradiative balance,
and therefore transport of heat from the tropicsto the poles is
required for the Earth system to be in globalradiative balance.
Identifying the mechanisms by which heat istransported from the
tropics to the poles is one of the centralproblems of climate
research. In addition, Rossby (5)drew attention to the
fact that because of its large specificheat capacity and mass, the
world ocean could store large amountsof heat and remove this heat
from direct contact with the atmospherefor long periods of time. The
results of these studies are thesubject of this research
article.

Until recently, little work has been done in systematically identifying ocean
subsurface temperature variability on basinand global scales, in
large part due to the lack of data [recentstudies include (

6-8)]. The first stepin examining the role of the ocean in
climate change is to constructthe appropriate databases and analysis
fields that can be usedto describe ocean variability. About 25 years
ago, ship-of-opportunityprograms were initiated to provide
measurements of subsurfaceupper ocean temperature. Before the
initiation of these programs,subsurface oceanographic data were not
reported in real time,as is the case with much meteorological data.
During the past10 years, projects have been initiated (9)
that haveresulted in a large increase in the amount of historical
upperocean thermal data available to examine the interannual
variabilityof the upper ocean. Using these data, yearly, objectively
analyzed,gridded analyses of the existing data were prepared and
distributed(7) for individual years for the
period 1960 to 1990.We have used the recently published World
Ocean Database 1998(10-13) to prepare yearly andyear-season objectively analyzed temperature anomaly fields. Detailedinformation about the temperature data used in this study canbe found in this series. Computation of the anomaly fields wassimilar to our earlier work (7), but some procedureswere changed (7).

To estimate changes in heat content at depths greater than 300 m, we prepared
objective analyses of running 5-year compositesof all historical
oceanographic observations of temperature forthe period 1948 to 1996
at standard depth levels from the surfacethrough 3000-m depth using
the procedures described above. Constructingcomposites of deep-ocean
data by multiyear periods is necessarydue to the lack of deep-ocean
observations. Most of the data fromthe deep ocean are from research
expeditions. The amount of dataat intermediate and deep depths
decreases as we go back furtherin time.

Temporal Variability of Upper Ocean Heat Content

Figure 1 shows the variability of yearly heat content anomalies in
the upper 300 m for 1948 to 1998 for individualocean basins defined
using the Equator as a boundary. Each yearlyestimate includes the
standard error of the mean anomaly valuefor each year plotted as a
vertical bar. The anomaly fields forthe Atlantic and Indian oceans,
for both the entire basins andNorthern and Southern Hemisphere
basins of each ocean, show apositive correlation. In each basin
before the mid-1970s, temperatureswere nearly all relatively cool,
whereas after the mid-1970s theseoceans are in a warm state. The
year of largest yearly mean temperatureand heat content for the
North Atlantic is 1998. In 1998 heatcontent reaches a value of ~4 ×
1022 J, equivalent to a volume mean temperature anomaly of
0.37°C.[Expanded versions of Figs. 1 and 4 with
volumemean temperature scales as well as heat content scale and
similartime series for heat content integrated through 1000-m depth
canbe viewed at Science Online (14) as Web figures 1
to3.]

Both Pacific Ocean basins exhibit quasi-bidecadal changes in upper ocean heat
content, with the two basins positively correlated.During 1997 the
Pacific achieved its maximum heat content. A decadal-scaleoscillation in North Pacific sea surface temperature (PacificDecadal Oscillation) has been identified (

15, 16),but it is not clear if the variability we observe in
Pacific Oceanheat content is correlated with this phenomenon or
whether thereare additional phenomena that contribute to the
observed heatcontent variability.

In order to place our results in perspective, we compare the range of upper
ocean heat content with the range of the climatologicalannual cycle
of heat content for the Northern Hemisphere and worldocean computed
as described by (

8) but using a morecomplete oceanographic database (10-13).There is
relatively little contribution to the climatologicalrange of heat
content from depths below 300 m. Our results indicatethat the
decadal variability of the upper ocean heat content ineach basin is
a significant percentage of the range of the annualcycle for each
basin. For example, the climatological range ofheat content for the
North Atlantic is about 5.6 × 1022 J, and the interdecadal range of
heat content is about 3.8 ×1022 J.

Changes in Temperature at 1750-m Depth in the North Atlantic
Ocean

Figure 2, A and B, shows the changes of temperature at a depth of
1750 m for 1970-74 minus 1955-59 (Fig. 2A)and for 1988-92
minus 1970-74 (Fig.
2B). The differencefield for
the two earlier periods shows that much of the NorthAtlantic was
warming between these periods, with the exceptionof a region of
cooling associated with the Mediterranean Outflow(17,
18). The difference field between thelater two pentads
demonstrates the opposite picture. The subarctichas cooled, with the
magnitude of maximum changes exceeding 0.4°Cin the Labrador Sea.

Parts of the midlatitudes and subtropicalregions have also cooled
substantially. Maximum warming is associatedwith the tongue of
temperature associated with the MediterraneanOutflow. The changes in
salinity at this depth (not shown) inboth sets of pentadal
differences are positively correlated withthe changes in
temperature, with the result that these changesin temperature and
salinity are at least partially density compensating.Tests of
statistical significance (Student's t test) have beenperformed on these difference fields, and we find (not shown)that the changes over most of the North Atlantic are statisticallysignificant, as was found for the earlier pentadal differences(

18). The observed changes are not small and can makean appreciable contribution to Earth's heat balance on decadaltime scales, which we quantify in the next section.

Heat Storage of the North Atlantic

Figure 3, A and B, shows the heat storage (computed as the time
derivative of heat content) for the 0 to 300 m (Fig. 3A)and
0 to 3000 m (Fig.
3B) layers of the North Atlanticbetween the 1970-74 and 1988-92 pentads (using the midpoints ofthe two pentads to compute the time difference between periods).This figure clearly indicates that maximum heat storage for thisbasin occurs at depths exceeding 300 m.

Cooling occurred throughout the subarctic gyre, with the maximum heat storage
exceeding 6 W min the Labrador Sea. Warming occurred in the midlatitudes
andsubtropics, with values exceeding 8 W min the midlatitudes of the
western North Atlantic. We have computedthe contribution to the
vertically integrated field shown in

Fig. 3Bfrom each 500-m
layer of the North Atlantic. The cooling of theLabrador Sea
is from each 500-m-thick ocean layer down
to 2500-m depth. The warmingin the western midlatitudes is due to
nearly equal contributionsby the 0- to 500- and 500- to 1000-m
layers, with some small contributionsfrom deeper layers. The warming
associated with the MediterraneanOutflow is mainly due to
contributions from the 1000- to 2000-mlayer.

There is a consistent warming signal in each ocean basin, although the
signals are not monotonic. The signals between theNorthern and
Southern Hemisphere basins of the Pacific and Indianoceans are
positively correlated, suggesting the same basin-scaleforcings. The
temporal variability of the South Atlantic differssignificantly from
the North Atlantic, which is due to the deepconvective processes
that occur in the North Atlantic. Beforethe 1970s, heat content was
generally negative. The Pacific andAtlantic oceans have been warming
since the 1950s, and the IndianOcean has warmed since the 1960s. The
delayed warming of the IndianOcean with respect to the other two
oceans may be due to the sparsityof data in the Indian Ocean before
1960. The range of heat contentfor this series is on the order of 20
× 1022 J for the world ocean.

Discussion

Our results demonstrate that a large part of the world ocean has exhibited
coherent changes of ocean heat content during thepast 50 years, with
the world ocean exhibiting a net warming.These results have
implications for climate system research andmonitoring efforts in
several ways. We cannot partition the observedwarming to an
anthropogenic component or a component associatedwith natural
variability. Modeling studies are required even tobe able to attempt
such a partition. However, our results supportthe findings of Hansen
et al. (

19), who concluded thata
planetary radiative disequilibrium of about 0.5 to 0.7 W mexisted for the period 1979 to 1996 (with the Earth system
gainingheat) and suggested that the "excess heat must primarily be
accumulatingin the ocean." Hansen et al. included estimates
of the radiativeforcings from volcanic aerosols, stratospheric ozone
depletion,greenhouse gases, and solar variability. Such information
is criticalfor studies attempting to identify anthropogenic changes
in Earth'sclimate system. This is because coupled air-sea general
circulationmodel experiments that are used to assess the effects of
increasingcarbon dioxide frequently begin integration with a sudden
increaseof atmospheric carbon dioxide (e.g., twice the present
value)rather than the gradual buildup observed in nature. This is
doneto minimize computer time required for completion of the
timeintegrations of these numerical experiments. Integration in
thismanner introduces what is known as a "cold start" error
(20,21).

Global sea surface temperature time series (

1) for the past 100
years show two distinct warming periods. The firstoccurred during
the period 1920 to 1940 and was followed by aperiod of cooling; the
second warming began during the 1970s.It is important to note that
the increase in ocean heat contentpreceded the observed warming of
sea surface temperature. It isnot clear what physical mechanisms may
be responsible for theobserved increase in ocean heat content. The
warming could bedue to natural variability, anthropogenic effects,
or more likelya combination of both. It may seem implausible that
subsurfaceocean warming preceded the observed global mean warming of
surfaceair and sea surface temperature. This phenomenon is possible
becausethe density of sea water is a function of salinity as well
astemperature. Thus, relatively warm and salty water or cold
andfresh water can reach subsurface depths from a relatively
smallregion of the sea surface through the processes of
convectionand/or subduction and can then spread out and warm or
freshena much larger region such as an entire gyre or basin. This
isclearly occurring in the North Atlantic Ocean by the
mechanismof deep ocean convection (Fig. 2). Lazier
(22)has documented the cooling and freshening of
the deep LabradorSea that began with the renewal of deep convection
in the early1970s. Dickson et al. (23)
have related the renewalof convection in the Labrador Sea to the
North Atlantic Oscillation(NAO) in sea-level pressure.

Nerem et al. (

24) showed for the period 1993 to
1998 that a relative maximum in global mean sea level and sea surfacetemperature [based on TOPEX/Poseidon altimetric measurements andthe Reynolds sea surface temperature analyses (25)]occurred at the beginning of 1998. This was associated with
theoccurrence of El Ñino. Global sea level began decreasing
duringthe rest of 1998. Part of the reason for extreme values in
NorthAtlantic heat content observed during 1998 may be related to
the1997 El Ñino, but additional analyses are required to
understandthe large increase in the North Atlantic heat content
between1997 and 1998. In addition, we emphasize that the extreme
warmthof the world ocean during the mid-1990s was in part due to a
multidecadalwarming of the Atlantic and Indian oceans as well as a
positivepolarity in a possible bidecadal oscillation of Pacific
Oceanheat content.

One possible link between the Northern Hemisphere oceans and the atmosphere
may be found in recent research culminating inthe publication by
Thompson and Wallace (

26). Theirwork indicates
that the NAO may in fact be part of a hemisphericmode of sea-level
pressure termed the Arctic Oscillation. Theseauthors also relate
changes at sea level associated with the NAOto changes at the 500-mb
height of the atmosphere. Recently, otherinvestigators have related
changes in the Northern Hemisphericstratospheric circulation to
tropospheric changes related to theNAO pattern (27-31). Dicksonet al. (23) have correlated
convection in the LabradorSea with the polarity of the NAO. To the
extent that these relationsare found to be statistically
significant, it may be that changeswe observe in global ocean heat
content may be related to thehemispheric and/or global modal
variability of the atmosphere,from sea level through the
stratosphere. Determining such possiblelinks is a major part of
understanding the mechanisms that governthe state of Earth's
climate.

Our final point relates to the large change in Atlantic heat storage from
depths exceeding 300 m. Because convection can resultin mixing of
water through the entire 2000-m depth of the watercolumn in the
Labrador Sea, changes in sea surface temperaturemay remain
relatively small in this region despite a large heatflux from ocean
to atmosphere. This flux is responsible for thelarge changes of heat
content we have documented at 1750-m depth.This may be an important
consideration when comparing the relativerole of the tropics and
high-latitude convective regions in effectingclimate change, whether
due to natural or anthropogenic causes.

REFERENCES AND NOTES

Intergovernmental Program on Climate Change, Climate Change 1995: The
Science of Climate Change, the Contribution of Working Group 1 to the Second
Assessment Report of the Intergovernmental Panel on Climate Change
(Cambridge Univ. Press, Cambridge, UK, 1996).

S. Levitus, T. P. Boyer, J. I. Antonov, World Ocean Atlas 1994,
vol. 5, Interannual Variability of Upper Ocean Thermal Structure. NOAA
Atlas NESDIS 5 (U.S. Government Printing Office, Washington, DC, 1994).
Temperature anomaly values at each standard level in each temperature profile
were computed by subtracting the climatological temperature value (17) for the
month in which the profile was measured. Data values exceeding 3 standard
deviations (SDs) from the 5° square in which the data occurred were not used
to create the climatology. One notable difference between the procedure used
to construct the anomaly fields presented in this study and our earlier work
(7) is that we used a 6-SD check to flag data as not being usable in this
study as compared to the 3-SD check used earlier. This change was made after
studies indicated that changes in upper ocean temperature associated with data
in the 3- to 6-SD range were real. We computed anomaly fields using all data
and compared these results with computed fields on the basis of data that did
not include values exceeding 3 SDs. After comparing these two sets of fields,
it became clear that large-scale anomaly features were being reduced in
amplitude by the 3-SD criterion. Therefore we used a 6-SD criterion to flag
data for elimination in our anomaly field computations and manually identified
and flagged data that created unrealistic small-scale features (typically
bull's-eye patterns with wavelengths less than 500 km). For each year or
year-season compositing period, all anomaly values were averaged in each 1°
square at each standard depth level from the surface to 1000-m depth. Anomaly
fields for each compositing period at each standard depth level were created
using a first-guess equal to zero at each grid point. Use of zero as a
first-guess rather than persistence or some other procedure minimizes the
creation of spurious anomalies. Objective analysis of the 1° square anomaly
means was performed with the same analysis procedures as given by (17).

The construction of the analyses shown in this work was supported by
grants from the NOAA Climate and Global Change program. Preparation of the
databases used in this work was supported by the NOAA and NOAA/NASA Climate
and Global Change programs and the NOAA ESDIM program. J.I.A. is a University
Corporation for Atmospheric Research (UCAR) Project Scientist at NODC/NOAA.